<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        function fn (prev, current) {
            return prev + current;
        }
        Array.prototype.myReduce = function (getNext, initValue) {
            for (const item of this) {
                initValue = getNext(initValue, item);
            }
            return initValue;
        }
        const arr = [1, 2, 3, 4];
        console.log(arr.myReduce((fn), 0));
    </script>
</body>
</html>